The corrosion resistance of Haynes 230, Hastelloy S and X hightemperature alloys and Hastelloy N, B-3, G-35, C-2000 corrosionresistant alloys was investigated in a wide temperature range (450-650 °C) in fused KCl-AlCl 3 mixtures. It was found that the mechanisms of corrosion of high-temperature alloys and corrosionresistant alloys in KCl-AlCl 3 based melts are different. At a critical temperature phase structure of the high-temperature alloys changes after contact with chloroaluminate melts resulting in formation of intermetallic or carbon-containing phases along the grain boundaries that result in increasing strength of the alloys but initiate intense intergranular corrosion. The structural changes in most of the corrosion-resistant alloys take place at higher temperatures but they also can cause structural changes in the materials. It was shown that both types of alloys undergo intergranular corrosion up to the critical conditions. Formation of different secondary phases was detected and their influence on the corrosion processes mechanism is analyzed.
The corrosion behavior of the corrosion-resistant alloy Hastelloy G-35 (manufactured by Haynes International, Inc.), corrosion and heat resistant alloy VDM Alloy 600 or Nicrofer 7216 and corrosion-resistant alloys VDM Alloy C-4 or Nicrofer 6616 and VDM Alloy 625 or Nicrofer 6020 (all produced by VDM Metals) was studied at 450-650 °C in fused KCl-AlCl3 mixture with the initial AlCl3-to-KCl ratio of 1.1. Time of exposure varied from 6 to over 1000 h. The corrosion rates of all the nickel-based alloys studied were determined by the red-ox processes resulting in dissolving the most electronegative alloy components (Cr, Fe and Mn) indicating that the processes taking place had electrochemical nature. Increasing temperature led to a noticeable increase of corrosion rates and a change of the corrosion process nature. Transmission electron microscopy revealed that intermetallic phases (such as sigma-phase in case of Hastelloy G-35 and Alloy 625 or Ni2(Cr,Mo) secondary phase in VDM Alloy C-4) can be formed during prolonged high-temperature exposure. These phenomena can accelerate the processes of intergranular corrosion and stress corrosion cracking of studied materials in industrial conditions. The results obtained agreed well with thermodynamic analysis, mechanical and thermophysical properties of the alloys and constructed "time-temperature-precipitation" diagrams.
Приведены результаты сравнительного исследования коррозионной стойкости алюмоматричного композита, полученного методом продувки кислородом предварительно гидрогенизированного расплава на основе сплава Al-Si-Fe с содержанием железа более 1 %, предназначенного для литья под давлением, и сплава Al-7Si с 0,3 % Fe, модифицированного лигатурой 5Al-Ti в количестве 2 %. Коррозия в алюминиевых сплавах обусловлена нарушением сплошности оксидной пленки на некоторых фазах, прежде всего на фазе Al 5 SiFe. Пары образцов из композита и сплава сравнения диаметром 15 мм и длиной 50 мм подверглись испытаниям в 7 %-ном растворе солевого тумана NaCl в камере КСТ-1 на подвесках при температуре 22 °С в течение 300 ч. Полученные результаты показали близкие значения убыли массы образцов, несмотря на значительно более высокое содержание железа в материале, поскольку сформировавшиеся в расплаве при продувке кислородом частицы Al 2 O 3 размером 100-200 нм, осаждаясь на границах фаз, снижают площадь поверхности взаимодействия с коррозионной средой. Литературные данные показывают существенное отличие сопротивления коррозионному воздействию композитов ex situ от in situ вследствие различий в размерах и расположении в матрице упрочняющей фазы. Исследованный композиционный материал может быть рекомендован как коррозионно-стойкая альтернатива сплавам с повышенным содержанием железа, используемым для литья под давлением.
The corrosion resistance of ferritic (08Kh17T) and ferriticmartensitic (12Kh13) type steels was investigated at 750 0 C in NaCl-KCl-VCl 2 melts. The mechanism of corrosion of 12Kh13 ferritic-martensitic steel in NaCl-KCl-VCl 2 melts includes two parallel processes: formation of new excessive phases inducing Fe 3 C (σ-phase)|melt|steel microgalvanic pairs, and reaction of most electronegative steel components (Mn and Cr) with V 3+ ions formed due to disproportion of V 2+ . Contacting samples of 08Kh17T with NaCl-KCl-VCl 2 melts resulted in no structural changes in the steel samples. In this case the mechanism of corrosion involves only one stage: disproportion reaction of V(II) with the formation V-Fe alloy and V 3+ ions and further oxidation of chromium in the steel by V(III). It was shown, that the presence of a suitable reducing agent in the contact with the melt (for example, metallic vanadium) can prevent the steel destruction.
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